Janus (star)

Double-faced white dwarf star in the constellation Cygnus

ZTF J203349.8+322901.1  Observation data
Epoch J2000 Equinox J2000

Constellation | Cygnus
Right ascension | 20h 33m 49.805s[1]
Declination | +32° 29′ 01.21″[1]
Characteristics
Evolutionary stage | White dwarf
Astrometry
Proper motion (μ)|  RA: 3.479 mas/yr[1]
Dec.: -6.971 mas/yr[1]
Parallax (π)| 2.4525±0.4364 mas[1]
Distance| approx. 1,300 ly
(approx. 410 pc)
Details[2]
Mass| 1.2 to 1.27 M☉
Radius| 3400+700
−600 km
Temperature| 34900+1300
−1500 K (hydrogen side)
36700+1300
−1600 K (helium side) K
Rotation| 14.97 minutes
Other designations

* Janus
* Gaia 1863529616173400576
* ZTF J203349.8+322901.1
* ZTF J2033+3229

Database references
SIMBAD| data

Janus, also known by its name ZTF J203349.8+322901.1, is a transitioning white dwarf located more than 1,300 light-years (400 pc) away[3] in the constellation Cygnus, discovered in 2019 by the Zwicky Transient Facility (ZTF), located at the Palomar Observatory, while looking for periodically variable white dwarfs.[2] Subsequent observations using the Low-Resolution Imaging Spectrometer (LRIS) on the W. M. Keck Observatory discovered its two-faced nature, with one hemisphere dominated by hydrogen, and the other dominated by helium.[4]

## Properties

[edit]

ZTF J203349.8+322901.1 is located more than 1,300 light-years (400 pc) away in the constellation Cygnus, with a mass between 1.2 M☉ and 1.27 M☉ (1.21 for an oxygen–neon core and 1.27 for a carbon–oxygen core), a radius of 3400+700
−600 km and a surface temperature of approximately 35,000 Kelvin. It is rare in that it has two hemispheres of different gases, one dominated by hydrogen, and the other dominated by helium. Another star, GD 323, shares this feature, albeit much more subtle.[2]

Janus' rotation period was observed by using CHIMERA, a high-speed imaging photometer, and HiPERCAM, a quintuple-beam imager (data collected on the nights of 6 and 9 September 2021 for a total of 2.1 hours), both located on the Gran Telescopio Canarias, which revealed a period of 14.97 minutes, which is much faster than what is usually observed in white dwarfs (hours to days). As it rotates, its spectrum transitions from only hydrogen lines to only helium lines at phases ≈ 0 and 0.5, respectively. There was no Zeeman splitting observed. The two hemispheres were measured to be at different temperatures, with the hydrogen side at 34900+1300
−1500 K and the helium side at 36700+1300
−1600 K.[2][5]

## Theories

[edit]

There are some theories as to why ZTF J203349.8+322901.1's hemispheres are so starkly defined and of different compositions.[2][6][5]

The first theory revolves around the belief that white dwarfs undergo an evolutionary phase, where helium sinks towards the bottom and hydrogen rises towards the top due to their masses. It is theorised that Janus was observed in an intermediate stage of this phase, and is exiting the DB gap on its way to becoming a DB white dwarf.[2][5]

The second theory is based on asymmetric magnetic fields:[2] If one hemisphere has a stronger magnetic field than the other, then the magnetic pressure at the pole will be higher, causing the hydrogen to diffuse towards the pole due to the ion pressure gradient, requiring a magnetic field of at least tens of kGs,[5] and less than a few MG.[2]

The third theory revolves around ZTF J203349.8+322901.1 being the result of a merger of two white dwarfs, due to its large mass and short rotation period.[2][5]

## References

[edit]

1. ^ a b c d Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
2. ^ a b c d e f g h i Caiazzo, Ilaria; Burdge, Kevin B.; Tremblay, Pier-Emmanuel; Fuller, James; Ferrario, Lilia; Gänsicke, Boris T.; Hermes, J. J.; Heyl, Jeremy; Kawka, Adela; Kulkarni, S. R.; Marsh, Thomas R.; Mróz, Przemek; Prince, Thomas A.; Richer, Harvey B.; Rodriguez, Antonio C.; van Roestel, Jan; Vanderbosch, Zachary P.; Vennes, Stéphane; Wickramasinghe, Dayal; Dhillon, Vikram S.; Littlefair, Stuart P.; Munday, James; Pelisoli, Ingrid; Perley, Daniel; Bellm, Eric C.; Breedt, Elmé; Brown, Alex J.; Dekany, Richard; Drake, Andrew; Dyer, Martin J.; Graham, Matthew J.; Green, Matthew J.; Laher, Russ R.; Kerry, Paul; Parsons, Steven G.; Riddle, Reed L.; Rusholme, Ben; Sahman, Dave I. (14 August 2023). "A rotating white dwarf shows different compositions on its opposite faces" (PDF). Nature. 620 (7972): 61–66\. arXiv:2308.07430. Bibcode:2023Natur.620...61C. doi:10.1038/s41586-023-06171-9. PMID 37468630. S2CID 259993565. Retrieved 9 March 2024.
3. ^ Cooper, Keith (19 July 2023). "Two-faced white dwarf star leaves astronomers puzzled". Physics World. Retrieved 11 March 2024.
4. ^ Devlin, Hannah (20 July 2023). "Two-faced star with helium and hydrogen sides baffles astronomers". The Guardian. Retrieved 9 March 2024.
5. ^ a b c d e Moss, Adam; Bergeron, P.; Kilic, Mukremin; Jewett, Gracyn; Brown, Warren R.; Kosakowski, Alekzander; Vincent, Olivier (12 December 2023). "Discovery of a magnetic double-faced DBA white dwarf". Monthly Notices of the Royal Astronomical Society. 527 (4): 10111–10122\. arXiv:2312.05749. doi:10.1093/mnras/stad3825. Retrieved 11 March 2024.
6. ^ Ravisetti, Monisha (21 July 2023). "Strange two-faced dying star 'Janus' baffles scientists in cosmic oddity". Space.com. Retrieved 9 March 2024.

* v
* t
* e

Constellation of Cygnus

* List of stars in Cygnus

Stars|

| Bayer|

* α (Deneb)
* β (Albireo)
* γ (Sadr)
* δ (Fawaris)
* ε (Aljanah)
* ζ
* η
* θ
* ι1
* ι2
* κ
* λ
* μ
* ν
* ξ
* o1
* o2
* ο3
* π1 (Azelfafage)
* π2
* ρ
* σ
* τ
* υ
* φ
* χ
* ψ
* ω1
* ω2
* P
* Q

|

Flamsteed|

* 2
* 4
* 8
* 9
* 15
* 16 (c)
* 17
* 20 (d)
* 22
* 23
* 26 (e)
* 27 (b1)
* 28 (b2)
* 29 (b3)
* 33
* 35
* 39
* 41
* 47
* 52
* 55
* 56
* 57
* 59 (f1)
* 61
* 63 (f2)
* 68 (A)
* 71 (g)
* 72
* 74
* 75

Variable|

* R
* T
* W
* X
* Y
* RW
* SS
* SU
* TT
* AZ
* BC
* BI
* CH
* KY
* V380
* V389
* V404
* V476
* V1027
* V1057
* V1143
* V1191
* V1331
* V1334
* V1489
* V1500
* V1668
* V1794
* V1974
* V2214
* V2513

HR|

* 7633
* 7767
* 7912
* 8193

HD|

* 185269
* 185351
* 187123
* 188753
* 190655
* 191612
* 191806
* 197037

Gliese|

* 777
* 806
* 1245

Kepler|

* 2
* 3
* 5
* 6
* 11
* 15
* 16
* 17
* 18
* 22
* 23
* 27
* 28
* 29
* 31
* 32
* 33
* 34
* 35
* 36
* 39
* 40
* 41
* 42
* 44
* 45
* 47
* 48
* 51
* 56
* 61
* 63
* 66
* 67
* 68
* 69
* 70
* 78
* 80
* 84
* 86 (PH2)
* 87
* 89
* 107
* 167
* 182
* 186
* 223
* 289
* 371
* 385
* 411
* 419
* 432
* 445
* 452
* 737
* 1229
* 1520
* 1625
* 1649

WR|

* 133
* 134
* 135
* 136
* 137
* 138a
* 140
* 142
* 147
* 148
* 150

Other|

* AFGL 2591
* BD+40°4210
* BD+43 3654
* Cygnus OB2 #8A
* Cygnus OB2 #12
* Cygnus X-1
* Cygnus X-3
* G79.29+0.46
* GSC 03949-00967
* HAT-P-17
* KELT-20
* KIC 8462852
* KIC 9832227
* KIC 9970396
* KIC 11026764
* KIC 11145123
* KOI-5
* KOI-74
* KOI-81
* MWC 349
* N6946-BH1
* PSR J2032+4127
* SPECULOOS-3
* W75N(B)-VLA2
* WASP-48
* WISE J2000+3629
* Wolf 1069

Star
clusters|

| Association|

* Cygnus OB2
* Cygnus OB7
* Cygnus OB9

|

Open|

* Berkeley 86
* DR 6
* IC 5146
* Messier 29
* Messier 39
* NGC 6811
* NGC 6819
* NGC 6834
* NGC 6866
* NGC 6871
* NGC 6910
* NGC 7031

Molecular
clouds|

* Cygnus Molecular Nebula Complex
* Cygnus X (including DR 21)

Nebulae|

| Dark|

* Barnard 146
* Barnard 147
* IC 5146
* L1014

|

H II|

* NGC 6914
* North America Nebula
* Pelican Nebula
* Sadr Region
* Sh2-101
* Sh2-106

Planetary|

* Abell 78
* Egg Nebula
* IRAS 19475+3119
* IRAS 20324+4057
* Kronberger 61
* M1-92
* NGC 6826
* NGC 6881
* NGC 6884
* NGC 7008
* NGC 7026
* NGC 7027
* NGC 7048
* Soap Bubble Nebula

WR|

* Crescent Nebula

SNR|

* Cygnus Loop (including Veil Nebula)

Galaxies|

| NGC|

* 6801
* 6946
* 7013

|

Other|

* 3C 438
* 4C +48.48
* Cygnus A
* QSO B1954+513
* QSO B2005+403
* ZTF J203349.8+322901.1

Exoplanets|

| Kepler|

* 2b
* 3b
* 3c
* 5b
* 6b
* 11b
* 15b
* 16b
* 17b
* 22b
* 23b
* 28b
* 32b
* 33b
* 34b
* 36b
* 39b
* 40b
* 41b
* 47b
* 56b
* 61b
* 64b (PH1b)
* 68b
* 69b
* 70b
* 78b
* 87c
* 186b
* e
* f
* 371b
* c
* 409b
* 419b
* 432b
* 451b
* 452b
* 560b
* 737b
* 1229b
* 1520b
* 1625b
* 1652b
* 1658b
* 1649b
* c
* 1708b

|

Other|

* b3 Cygni b
* HD 185269 b
* HD 187123 b
* c
* HD 191806 b
* KELT-9b
* KELT-20b
* TrES-5b

Exomoons|

| Kepler|

* 1625b I
* 1708b I

|

*[RA:]: Right Ascension
*[Dec.:]: Declination
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